256 HISTORY OF SCIENCE. 



very large reward to any one who should discover a method of finding 

 the longitude at sea : ; 10,000 was the reward if the longitude were 

 found within a degree ; ^i 5,000 if within forty minutes ; and ,20,000 

 if within half a degree. These munificent rewards had the desired 

 effect. JOHN HARRISON, a man of humble origin but great genius, had 

 devoted his lifetime to the construction of clocks and watches for navi- 

 gators. The improvements he made in the marine chronometer were 

 so numerous and important that we may consider him the inventor of 

 the instrument. For these improvements he obtained a medal from 

 the Royal Society in 1749 ; and in 1761 the Board of Longitude per- 

 mitted a trial of his chronometer to be made by a voyage to Jamaica. 

 At the end of sixty-one days Harrison's chronometer gave the longitude 

 of Port Royal within 5" of time ; and, on the return to England, after an 

 absence of a hundred and sixty-one days, the whole variation was only 

 i' 5". The condition of the Act of Parliament had been satisfied, and 

 Harrison received within two or three years the whole reward of 

 ^20,000. From this time the use of chronometers spread rapidly 

 among navigators ; but the sextant has also always maintained its posi- 

 tion as the essential instrument for the seafarer. The advantage is 

 obvious of having two independent methods for the longitude, one of 

 which shall act as a check upon any accidental errors of the other. 



In 1736 Bouguer and La Condamine endeavoured to measure the 

 gravitative attraction of the mountain Chimborazo in Peru. In a sta- 

 tion north of the mountain its attraction causes the plumb-line of the 

 sector to deviate from the true vertical towards the south, and the con- 

 verse is true on the south side. Hence the apparent latitudes found 

 at each station differ from the real, and the difference between these 

 latitudes will be the sum of the attractions of the mountain at the two 

 stations. If, therefore, the true latitudes are previously determined by 

 other methods, such as trigonometrical surveys, it is easy to find the 

 attraction of the mountain by its perturbing effect on the plumb-line. 

 Bouguer did not, however, obtain very satisfactory results; but he, 

 indeed, did not operate under the conditions just stated, for both of 

 his stations were to the south of the mountain, and hence it was not 

 the sum, but the difference, of the attractions which was observed. The 

 experiment of observing the actual deflection of a plumb-line produced 

 by a neighbouring mountain was repeated under more favourable con- 

 ditions by Maskelyne, in 1 7 74, on the mountain of Schehallien in Perth- 

 shire. This mountain rises 2,000 feet above the sea-level, and it was 

 easy to select one station for observation to the south of the mountain 

 and another to the north. In this case it was found, after repeated 

 observations, that the sum of the attractions of the mountain at the 

 two stations amounted to 1 1*7". The mean density of Schehallien was 

 then determined by carefully finding the density (/.<?., specific gravity) 

 of its constituents, and estimating approximately the quantity of each 

 contained in the mountain. From the data thus obtained, the den- 



